Since this is a private git repository you should have an user and a password to use when you are cloning the repository. If you don't have them configured you should see a message like this one:
I think this case from Stackoverflow can be useful to you, in that case you have instructions to configure your mail/password or even a git token if your private repo works with one.
Please, do not hesitate on asking for details if you don't understand something, I will me more than glad to help you
I hope this information is useful for you.
Thank you and best regards,
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